When using PHP 4.2.0 even on the same page where you registered the variable with:

session_register("someVar");

if you try to see if the variable is set and do not assign it a value before, the function used in the previous comment will give the same output. This may show that the variable is declared and will not be set until some value is give assign to it. I think that this way will give the option to register all the variables used for sure on the process on the first page and using them as the time comes.